Catalyst components for the stereospecific polymerization of olefins, such as propylene, are widely known in the art and they are of the Ziegler-Natta category type. The first catalyst of this type widely used in the industry was based on the use of solid TiCl3 obtained by reduction of TiCl4 with aluminum alkyls. The activity and stereospecificity of the catalysts were not so high so that the polymer had to be subject to a deashing treatment to remove the catalyst residues and to a washing step for removing the atactic polymer produced. Nowadays, the most spread out catalyst family used industrially comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. The titanium atoms in the solid catalyst component have a +4 oxidation state. Conventionally these catalysts are used together with an external donor (for example an alkoxysilane) which helps in obtaining higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility. It is however of general interest the possibility of increasing the xylene insolubility particularly for catalyst components based on donors different from phthalates. These latter in fact generated some health concerns in the industry and, as a consequence, alternatives classes of donors are being evaluated. It has been generally noted that the alternative classes of donors tend to generate catalysts slightly less stereospecific which in propylene polymerization produces a lower amount of polymer fraction insoluble in xylene. In principle it is possible increasing the stereospecificity by increasing the amount of donor in the catalyst. However, as a downside, the polymerization activity decreases and the balance activity/stereospecificity worsens anyhow.
Based on this, it would be very convenient to find a way of improving the stereospecificity of a catalyst without decreasing its activity and in particular it would be convenient that this method be of a wide applicability.
Since the discovery of magnesium chloride based supports numerous attempts have been made to include in it additional compounds with the aim of imparting new or modified properties to the final catalysts.
In U.S. Pat. No. 4,613,655 substantial amounts (30% by weight or higher) of different inorganic compounds and, among them ZnO and ZnCl2, are mixed with MgCl2 and then ground in the presence of TiCl4 salts in order to produce a catalyst. Apart from the effect of dilution of MgCl2, the catalyst did not show any improvement in polymerization activity.
An increase in polymerization activity is reported by Fregonese and Bresadola (catalyst system supported on MgCl2 doped with ZnCl2 for olefin polymerization—Journal of Molecular Catalysis A: Chemical 145 (1999) p. 265-271) that discloses the preparation of a catalyst support by mixing metallic Mg and up to 2% of metallic Zn and subsequent reaction with n-BuCl. The so obtained support contains MgCl2 in δ-form and ZnCl2 but also contains a substantial amount of metal-alkyl groups (Table 1). These latter, which appear reduced in amount after reaction with TiCl4 are responsible for the presence of titanium atoms in the reduced +3 oxidation state. In propylene and ethylene polymerization the catalyst shows an increased activity up to about 2% of Zn content; however, the effect on stereospecificity is minimal as the amount of boiling heptane insoluble fraction remains always below 90% and when the amount of Zn approaches 2% wt the heptane insoluble fraction dramatically drops to less than 80%.
On the other hand, higher amounts of ZnCl2 generates, according to CN 1631922, a catalyst, having a support containing mixed Mg—Zn halides in which ZnCl2 constitutes more than 15% based on the total of Mg—Zn halides on which are supported Ti atoms and an electron donor, capable of polymerizing vinyl aromatic monomers such as styrene. Interesting results in ethylene/styrene copolymerization are obtained only when the amount of ZnCl2 used in support preparation was higher than the amount of MgCl2.
Now the applicant has found that it is possible to increase the stereospecific response of catalyst component based on Mg containing support on which are supported titanium atoms and donors by modifying it with specific amounts of Zn compounds and by the proper selection of Ti species.